In many computer systems, the storage and retrieval of information for and by computer applications is handled by one or more central storage systems. For example, one type of storage system commonly used in personal computers is a file-folder-and-directory-based system, also termed a “file system.” Such file systems organize pluralities of files into hierarchies to create an abstraction of the physical organization of the storage medium used to store the files. Generally, such organization into a hierarchy occurs at the operating system level. The files stored generally include the file hierarchy itself (the “directory”) embodied in a special file maintained by the file system. This directory, in turn, maintains a list of entries corresponding to all of the other files in the directory and the nodal location of such files in the hierarchy (herein referred to as the folders).
The use of file system for central storage has several limitations. These may be overcome by using relational database technology as the underpinning of a central storage system, as described in the patent in the patent applications referenced above in the “Cross-Reference” section.
In an application using data stored in a storage system, some activity of the application may be predicated on changes to the data stored in the storage system. For example, an email program may wish to know when a new email message arrives. In certain systems, email messages arrive and are stored on the storage system. When a new message arrives, the email program can then perform any internal processing using the fact that a new email message has arrived or using the content of the email message. For example, some email programs contain rules used to process messages. The application of these rules is triggered, in some cases, by the arrival of a new message. Additionally, there may be user interface changes based on changes to data stored in the storage system. For example, upon the arrival of a new message, an alert may be presented to the user which indicates that a new message has arrived. A list of messages may be refreshed to include the new message.
In view of the foregoing, there is a need for a change notifier that can be used in a database-based storage system and provides for good performance and scalability. The present invention satisfies this need.